The Highly Active Antiretroviral Therapy (HAART), employing a three-drug regimen acting on different stages of the viral life cycle, has dramatically increased the survival rate of the HIV-infected individuals, and has turned the Acquired Immunodeficiency Syndrome (AIDS) into a controllable chronic illness. A fateful outcome of the chronic HIV condition, however, is the progressively weakening immune system since HIV primarily infects the CD4 lymphocytes which help the body fight infections. This makes the patients vulnerable to opportunistic co-infections including, but not limited to that caused by Hepatitis C virus (HCV). The end-stage liver diseases caused by hepatitis viral infection is now one of the major causes of death (>50%) in HIV patients. In a recent study exploring the cause of death in HIV patients, a majority of the dead had tested positive for antibodies to HCV. Out of the HIV opportunistic infections, HCV in particular has lately taken the center stage, and is causing alarms in the AIDS research community for many reasons, including (a) the vastly successful HAART therapy is considerably less effective with HIV patients co-infected with HCV, (b) the protease inhibitors used in the HAART therapy exert a significant degree of extra strain on the liver that is already stressed by HCV. This results in dramatic exacerbation of HCV and its accelerated progress to liver cirrhosis and death. Thus, patients on HAART therapy are even more at risk for liver disease, and (c) the HCV infection is believed to stimulate the HIV activity, for example, the increased HIV RNA levels and decreased CD4+ cell counts were found in HIV patients co-infected with HCV, and (d) the approved anti-HCV therapy with a combination of 1-interferon and ribavirin was shown to decrease the potency of anti-HIV therapy because of the perceived molecular interaction of ribavirin with the reverse transcriptase inhibitors such as AZT used in HAART, resulting in the latter's diminished effectiveness. For all these reasons, mutually compatible anti-HCV and anti-HIV drugs are urgently needed to combat HCV co-infection in HIV patients. These drugs should neither exacerbate the clinical manifestations of the co-infection nor diminish the efficacy or effectiveness of the therapy used for treatment of the original infection. We propose here to advance three novel classes of ring-expanded nucleoside (REN) analogues that show promise of further drug development for treating HIV/HCV co-infection. While members from all three classes have shown potent anti-HCV activity in vitro, those from the first two classes, were also found to possess dual anti-HCV and anti-HIV activities in vitro, with little or no toxicity. The anti-HIV activity of the two classes of compounds appears to arise from their respective inhibitory effect at two different stages of the viral life cycle, an early event for class I and a late event for class II compounds. We have carried out some preliminary mechanistic studies which show that compounds of all three classes are inhibitors of HCV NTPase/helicase, while those of class I also inhibit HIV Integrase. The work is currently in progress on elucidating the mechanism of anti-HIV activity of class II compounds. The dual anti-HCV/HIV action of compounds of classes I and II has implications for potential replacement of an HCV-aggravating protease inhibitor in the HAART therapy with an inhibitor of a novel mechanism of action that would not cause adverse effects on the liver in treating HIV patients co-infected with HCV. As there is no known human equivalent of HIV integrase, the chances of developing drug resistance for integrase inhibitors are also far less compared with the notoriously resistance-prone protease inhibitors. Inhibition at two different stages of the HIV viral life cycle is an additional attractive feature of class I & II inhibitors based on REN skeleton. [unreadable] . [unreadable] [unreadable] [unreadable]